In general, a generator's output power increases as the number of revolutions (rotating speed) of the rotor increases. For example, a large capacity wind power generation system inputs thereto a rotational energy of a propeller. Usually, a rotating speed of the propeller is significantly low. When such a propeller is directly connected to a generator for power generation, the size of the generator needs to be huge. There is known an approach in which, in order to reduce the size of the generator, the rotating speed of the propeller is increased by a mechanical speed-up device having a gear.
In recent years, the capacity of the wind power generation has increased and, therefore, the above-mentioned mechanical speed-up device may fail to meet a required power generation capacity.
There is known, as an approach for solving this problem, a technology disclosed in, e.g., Patent Document 1 that uses a magnetic gear to obtain speed-up effect. The magnetic gear is a technology comparatively similar to the generator, allowing integration of the magnetic gear and the generator. A rotating electrical machine having the magnetic gear speeds up a rotating magnetic field using the magnetic gear and generates power in a three-phase armature winding based on the rotating magnetic field.